.blend files 'magically' shrinking?
Reopening your file removes all datablock that have been created and then not used, nor marked as "Fake user".

I.e. you create two materials for an object, then assign only one of them.
The unused object remains as datablock with no users.
When you close the file and then reopen it, the unused datablock (i.e. the unused material) disappears.

I don't know if this is your case!

See Blenders manual section on: Garbage Collection